1. Field of the Invention
This invention pertains to ignition and sensing systems and more particularly to flame ignition and flame detecting or sensing systems. Even more particularly, the invention pertains to such systems having a spark type ignition.
2. Description of the Related Art
A gas pilot burner is a device used to create a stable pilot flame by combustion of a low flow rate (relative to the main burner) gaseous fuel-air mixture. The pilot flame is used to light a larger main burner, or a difficult to light fuel. Gas pilot designs normally include an ignition system and a flame detection system. The two most common types of ignition systems used in gas pilot burners are high tension (HT) and high-energy ignition (HEI). Flame detection is typically by a flame ionization detection (FID) system.
An HT flame ignition system typically utilizes a high voltage source and an HT spark plug or spark rod. The high voltage source provides high voltage, low current pulses. Often, such pulses will be 15 kV or greater and from about 10 to about 50 mA. HT systems create low amperage sparks that bridge an air gap created in a spark plug or between a spark rod and the grounded pilot frame. This spark is used to ignite the fuel-air mixture and, thus, generate the pilot flame. While this type of ignition can be low cost, it can be inconsistent when ignition conditions are not ideal. Moisture from steam or rain, contamination and heavy fuel can all generate ignition problems when using an HT system.
An HEI system typically utilizes a capacitive discharge exciter to pass large current pulses to a spark rod. The large current pulses are often greater than 1 kA. The spark rod or igniter probe for an HEI system is generally constructed using a center electrode surrounded by an insulator and an outer conducting shell over the insulator such that, at the ignition end of the spark rod, a high-energy spark can pass between the center electrode and outer conducting shell. HEI systems have the ability to maintain powerful high energy sparks in adverse conditions such as cold temperatures, heavy fuels (heavy gases or oils), contamination of the igniter plug with coking or other debris and moisture presence due to steam purging or rain.
For safety considerations, it is important that the ignition system ignites the fuel-air premix as soon as possible after the main fuel gas valve opens. It is also important that the flame ionization detection system registers the flame signal as soon as possible after the flame is established. Together, rapid ignition and flame detection help minimize the chance of explosion due to raw fuel being pumped into a burner. Typically, there is a burner management system (BMS) that controls the fuel and ignition systems while monitoring the flame ionization detection system. Often, the burner management system will give five seconds or less of fuel flow time before closing the fuel valve if flame is not proven. The window for ignition and detection is therefore very short.
Most prior HT ignition systems have used a combined HT and flame detection system wherein ignition must occur and then an electromechanical switch de-energizes the exciter and energizes the flame detector. This means ignition and detection are sequenced into two distinct time periods, each occupying a portion of the maximum limited allowable fuel valve open time window. HT or HEI systems allowing for simultaneous ignition and flame detection have relied on using completely separate ignition and detection systems. It would be beneficial to have a powerful ignition system, such as an HEI system, and a flame detection system that can operate simultaneously through the entire window where the flame detection system is an integral part of the HEI systems; that is, without utilizing completely separate ignition and detection systems.